Rf
ampl

ABSTRACT

IMPROVEMENT IN COMMUNICATIONS RECEIVERS OF THE TYPE HAVING AN INTERMEDIATE FREQUENCY PASSBAND SUBSTANTIALLY WIDER THAN THE BANDWIDTH OF THE RECEIVED SIGNAL, SUCH IMPROVEMENT INVOLVING GATING MEANS COMPARING THE ENERGIES IN FREQUENCY RELATED SEGMENTS OF THE RECEIVER PASSBAND AND PASSING AD RECEIVER OUTPUT ONLY SUCH SEGMENTS AS HAVING A DIFFERENT ENERGY LEVEL (CAUSED BY SIGNAL PRESENCE) THAN THE ENERGY LEVELS IN OTHER OF THE SEGMENTS (CAUSED BY NOISE ENERGY). SUCH GATING MEANS CAN ALSO BE EMPLOYED FOR SQUELCH CONTROL, WITH THE RECIVER TENDERED SENSITIVE WHEN THE ENERGY LEVELS IN THE PASSBAND SEGMENTS ARE SUBSTANTIALLY DIFFERENT (I.E. SIGNAL ENERGY IS PRESENT) AND SQUELCHED WHEN THE ENERGY LEVELS IN THE SEGMENTS ARE SUBSTANTIALLY THE SAME (I.E. ONLY NOISE ENERGY IS PRESENT)

Oct. 26, 1911 l Original Filed Jan. 7. 1964 KAHN SIGNAL `v`SELECTION ANDSQUELCH CONTROL IN WIDEBAND RADIO REGEIVERS 2 Sheets-Sheet 1 1,-/ TINVENTOR. LEONA/M AfA/#V Oct., Z6, 1911 L R, KAHN y SIGNAL SELECTION ANDSQUELCH GONTROLRIN WIDEBAND RADIO REcEIvERs 2 Sheets-Sheet l OriginalFiled Jan. 7. 1964 INVENTOR. LEGNA/Y0 K/q/-/V H77- WEVS m www A r ms mfwUMH ATA United States Patent O Matter enclosed in heavy brackets appearsin the original patent but forms no part of this reissue specification;matter printed in italics indicates tlie additions made by reissue.

7 Claims ABSTRACT OF THE DISCLOSURE Improvement in communicationsreceivers of the type having an intermediate frequency passbandsubstantially wider than the bandwidth of the received signal, suchimprovement involving gating means comparing the energies in frequencyrelated segments of the receiver passband, and passing as receiveroutput only such segments as have a different energy level (caused bysignal presence) than the energy levels in other of the segments (causedby noise energy). Such gating means can also be employed for squelchcontrol, with the receiver rendered sensitive when the energy levels inthe passband segments are substantially different (ie. signal energy ispresent) and squelched when the energy levels in the segments aresubstantially the same (i.e. only noise energy is present).

This invention relates to radio communications receivers, particularlywideband receivers used in mobile communications service where, becauseof frequency drift on the part of oscillators in the receiver or theassociated transmitter, or because of Doppler shift induced by relativemotion between the receiver and the transmitter, the bandwidth of thereceivers must be considerably wider than theoretically required by thesignals modulation characteristic.

Other features of this invention relate to improved squelch circuitoperation in wideband receivers, by means of which a receiver isautomatically and effectively muted durin'g pauses in signaltransmission, without sacrice in receiver sensitivity.

It is often necessary to utilize relatively wideband receivers toreceive narrowband signals. Common reasons for this manner of operationinclude the following:

(a) Transmitter or receiver stability is poor and therefore the receivermust be wide enough to accommodate substantial carrier drift.

(b) Receiver tuning accuracy is insufficient to insure proper centeringof the signal carrier.

(c) Variant relative motion occurs between the receiver and thetransmiter, or the medium is in motion, thus creating Doppler shift.This factor is especially important -in space communications.

(d) Plural transmitting stations, each operating in nominally the samechannel and each serving to transmit the same intelligence to differentgeographical areas, give rise to severe interstation interference orso-called echo effects in some geographical areas.

At the present time many communications services use receivers withbandwidths considerably wider than modulation analysis would indicate tobe necessary. For example, aeronautical air-to-ground links generallyprovide 30 to 40 kilocycles (kcs.) receiver bandwidth even though thesignal itself is a double-sideband amplitude modulated (AM) signal witha maximum of 3 kcs. modulation (6 kes. spectrum width). This is due tothe fact that the Re. 27,202 Reissued Oct. 26, 1971 ACC signal path orpaths may be subject to dissimilar environmental variations and thus thesystem must accommodate appreciable carrier dr-ift.

The use of a wider receiver bandwidth creates an increase in noise levelin the receiver. For thermal and shot noise, `the noise power is alinear function of bandwidth in the frequency range of interest to mostcomunications engineers. Moreover, impulse noise, which may be moreimportant in aircraft and other vehicular communications, increases asthe square of the bandwidth. Thus, there is an appreciable loss insignal-to-noise ratio due to the use of wideband receivers.

In addition to the signal-to-noise problem of the signal channel thesquelch control circuits are very important in mobile communicationsservices and the difficulty of differentiating between the signal andthe noise in order to operate the squelch circuit is a very severeproblem.

In the final analysis, under normal operating conditions, the squelchcircuit determines the sensitivity of the receiver. The receiveroperator adjusts the squelch threshold so as to not be annoyed by thenoise from the receiver. The natural tendency is to minimize annoyanceby maintaining the squelch threshold higher than necessary. Thisdesensitizes the receiver and therefore only relatively strong signalsare heard.

An even more diicult problem is that of making intelligible a signalwhich is weaker than an interfering adjacent channel signal. One aspectof the present invention is the provision of means enabling effectiveselection of a signal that is weaker than an interfering signal in anadjacent channel.

A further problem addressed by this invention is the problem ofsustained network reception by aircraft in ight. In order to cover largeareas, aeronautical radio stations are very densely placed around theUnited States, providing an air-to-ground and ground-to-aircommunications network. These network stations, although nominally onthe same channel, do not all operate on the same frequency but on somesix frequencies spaced approximately 6 to 7 kcs. apart. The advantage ofdoing this is that while an aircraft flies from one location to anotherit picks up one station after another transmitting the same intelligenceso that as one station fades out, another station will come in with astrong signal. The reason slightly spaced frequencies are used is sothat they do not interfere with each other and cause fading patterns.Thus, those stations which operate `on exactly the same frequency aregeographically spaced far apart suficiently so that while they areoperating on the same frequency the aircraft at no time receives anappreciable signal from both stations.

f This system for reducing interstation interference has one severeproblem, there is often an echo which is mainly due to the difference intime of arrival of the audio wave at the various transmitter locations.Time of arrival differences arise because both cable and microwave typesof transmission paths are used for the audio intelligence prior toground-to-air transmission, often with facilities being switched orinterchanged from time to time. The echo effect greatly degrades thespeech quality of the received signal because the listener hears two ormore signals many times, with the echo often being quite pronounced.Even more important is the fact that the echo effect almost completelydestroys data accuracy at reasonable data transmission speeds. Theimproved signal selection technique of the present inventionautomatically selects a stronger signal and greatly attenuates anyweaker signals, thus substantially obviating echo induced datainaccuracy.

One conventional method for improving signal-to-noise (S/N) andsignal-to-interference (S/l) ratios in wideband receivers employed toreceive relatively narrowband signals is to yimprove the frequencystability of receiving and 'ansmitting equipments so the bandwidth ofthe receiver an be correspondingly reduced. Frequency stabilizationenerally takes the form of the use of crystal oscillators avingtemperature controlled enclosures and the use of 'equency synthesizerswherein an output frequency is erived from one or more extremely stableoscillators by se of frequency dividers, frequency mixers, harmonicenerators, or other such devices.

However, such frequency stabilized equipment is genrally complex, bulkyand expensive. For these reasons, s well as others, most mobileequipments do not incororate such devices and relatively poor frequencystability a tolerated. Also, in the case of satellite or spacecomiunications systems, the correction of the Doppler shift rrors is avery complex problem requiring a precise nowledge of the relative motionbetween the receiver and 1e transmitter, making such correctionequipment inppropriate for many applications.

Concerning the aspects of the invention relating to imroved squelchoperation, the conventional method of de- :rmning whether a signal isbeing received is to measure utomatic volume control (AVC) voltage. Ifthis oltage is greater than a certain value (the squelch threshld) thenit is assumed that a signal is present. This techique has a very seriouslimitation and that is that it is ot possible to determine from a simplemeasurement of .VC level whether the incoming wave is predominantly gnalor predominantly noise. Generally the receiver oprator must make anadjustment of his equipment to set a ireshold point, above which levelthe incoming wave is :msidered to lbe predominantly signal.

The threshold adjustment must be made quite careilly because, if thethreshold setting is made too low, oise energy of itself will oftenoperate the receiver causlg annoyance and fatigue of the operator.However, if ie threshold level is set too high, weak signals will begnored and for practical purposes the sensitivity of the :ceiver isdegraded.

The optimum squelch level adjustment is hard to ehieve and must bealtered for variable conditions such s moving from a region of low noiselevel to one of high oise level, or vice lversa. Also, the skill of theoperator very important to the proper adjustment, making the peratingcharacteristics of the receiver very sensitive to perator capabilitiesand other subjective considerations.

In practice of the present invention, the reeciver passand, e.g. theintermediate frequency (IF spectrum, is :parated into a number offrequency related divisions or :gments by a parallel array of bandpassfilters or the ke. The various filter outputs are fed to gates such asiode detectors which automatically select only that part one or at timestwo adjacent filter outputs) of the receiver assband having thestrongest energy level. The other filter utputs which would, at a giveninstant, merely add noise nd interference (as from weaker signals) aredecoupled r blocked by operation of their respective gates, so form opart of the receiver output. In preferred forms of the ivention, ifselection of the next to strongest filter output required on occasion,such as when an interfering sigal happens to be stronger than a desiredsignal, a similar :t of gates is available to reject the strongest andselect nly the next to strongest filter output. If desired, this imetechnique can of course be extended to select only 1e third strongestfilter output, etc.

The technique of dividing the IF spectrum b-y use of andpass filters orthe like also provides an improved ianner of squelch circuit operation.It is well known that 1e spectrum characteristic of resistor noise(thermal oise), tube noise (shot noise), or transistor noise (shot ndthermal noise) is very flat, i.e. the spectral density f the noiseenergy is constant for relatively narrow bandfidths. Even in the case ofignition noise, the energy disfibution passband divisions or spectralcomponents would e equal for situations where the present invention isto e used. This is true because the ignition noise repetition 4 rate isgenerally very low, with the result that the spacing between spectralcomponents is relatively quite small and a large number of almost equalignition noise energy components pass through each of the bandpassfilters.

When a signal is not being received, all the bandwidth spectrum dividingfilters thus have approximately equal noise output levels. However, whena signal is received, this equality is upset. It is accordinglypossi-ble to produce a squelch control voltage which varies as afunction of whether or not the bandpass filter outputs are essentiallyequal. This is the operating factor upon which the squelch controlsystem of the present invention relies. It is to be noted that when anarrowband signal is present, and because the IF spectrum is segmentedby a number of bandpass filters, the signal-to-noise (S/N) ratio of theenergy within the filter passing the narrowband signal is greatlyimproved, as compared with the signal-to-noise ratios of the energies inthe other filters, so squelch circuit control responsive to a comparisonof the energy levels of the various filter outputs can be quitesensitive and is more accurately responsive to signal presence than isthe case when squelch control is effected by sensing the total energypresent in the passband.

To summarize certain of the characteristic objects and features of thepresent invention, its advantages include the following: improvement ofthe signal-to-noise ratio of a narrowband signal received by a widebandreceiver; improvement of the signal-to-interference ratio of a widebandreceiver when a narrowband signal and interference energy are separatedin frequency by a frequency difference greater than the frequencyspectrum of the narrowband signal; provision in a wideband receiver forreceiving narrowband signals of the capability of selecting from amongvarious signals at various strengths and with various small frequencydifferences within the receiver passband only the strongest such signal,or the next strongest signal, or the second strongest such signal, orthe third strongest such signal, etc.; provision in a wideband receiverof a mode of squelch circuit operation which can effectively distinguishbetween signal energy in only a part of the receiver passband and noiseenergy distributed substantially uniformly in the passband, with thesquelch sensitivity being directly related to signal energy level ratherthan total energy level; provision in a wideband receiver of squelchcontrol means not requiring careful threshold level adjustment;provision of squelch circuit control means capable of operating at verylow signal-to-noise ratios; and provision in a wideband receiver ofmeans by which the selectivity characteristics of the receiver can bequickly and simply altered to meet varying operating conditions.

These and other objects, features, characteristics and advantages of thepresent invention will be apparent from the following specificdescription of certain typical and therefore non-limitive forms thereof,taken together with the accompanying illustrations, wherein likenumerals refer to like components, and wherein:

FIG. 1 is a simplified block diagram of a superheterodyne type widebandreceiver embodying both the passband segment selection feature and thesquelch circuit control feature of the present invention;

FIG. 2 is a block and schematic diagram of a portion of the passbandsegment selection circuit of the receiver shown in FIG. l;

FIG. 3 is a graphical presentation of the spectral distribution of thearray of bandpass filter utilized in the passband segment selectioncircuit shown in FIG. 2;

FIG. 4 is a block-schematic presentation of the passband segmentselection circuit of the receiver shown in FIG. l, including a parallelarray of bandpass filters and gating means enabling optional selectionof a signal of any relative strength to the exclusion of other signalsin the passband, and further showing means deriving squelch circuitcontrol outputs from said passband filters;

FIG. is a simplified block-schematic diagram showing schematically atypical squelch circuit control arrangement characteristic of theinvention;

FIG. 6 illustrates a modified form of the invention, showing a typicalapplication thereof to frequency shift keying (FSK) type radio telegraphsignal reception; and

FIG. 7 is a schematic showing of the automatic threshold adjust circuitof the receiver shown at FIG. 6.

FIG. l shows in simplied block form a superheterodyne type receiverembodying the present invention, both as to its passband segmentselection aspects and as to its squelch circuit control aspects. In amanner conventional per se in wideband superheterodyne receivers, thereceive comprises an antenna 10 delivering an input 12 to radiofrequency (RF) amplifier 14, the output 16- from which goes to mixer 18along with an output 20 from local oscillator 22, with mixer output 24being fed to one or more sideband IF amplier stages designated at 26, aportion 28 of output 30 from the wideband IF amplifier section 26 beingfed to an AVC detector stage 32 from which feedback outputs 34 and 36are fedV to the RF amplifier 14 and the wideband IF amplifier section26. As also conventional, AVC detector stage 32 functions to regulatethe gains of the RF and IF amplifier stages 14 and 26 so as to produce asubstantially constant amplitude output 30l from the wideband IFamplifier section 26 over a considerable range of signal level at input12.

A portion 38 of the output 30 from wideband IF amplier section 26 is fedto a passband segment selection circuit, generally designated at 40, ofa design according to the present invention, as discussed in more detailbelow in connection with FIGS. 2, 3 and 4. Passband segment selectioncircuit 40 develops an audio frequency output which contains only a partof the energy of the receiver -IF passband. In the simplest form ofcircuit (FIG. 2), only that part of the passband is selected whichcontains the strongest signal. However, in the preferred form of circuit(FIG. 4), selection circuit 40 develops a strongest signal output asindicated at 42A, a second strongest signal output as indicated at 42B,and can also provide further progressively weaker signal outputs ifdesired, a weakest signal output being shown at 42n in FIG. l, forpurposes of illustration in this respect.

Whichever of the signal outputs 42A, 42B, 42n is desired as the receiveroutput is selected by manual control of multi-position switch S1 andfrom there delivered as input 44 to one or more audio frequency (AF)amplification stages generally designated at 46, the output 48 fromwhich is applied across load resistances 50, 52, said resistor 52 beingthe squelch load and the resistors 50 and 52 constituting the fullsensitivity load in the squelch circuit, the nature of the output beingdetermined by the position of squelch relay contact S2 (shown in FIG. 1in its squelch off or receiver operative position). The audio signaloutput selected by said squelch contact S2 is then applied as an input54 to one or more additional AF amplification stages, generallydesignated at `56, from whence an output 58 is fed to suitable audiosignal reproduction means such as speaker 60.

The passband segment selection circuit 40 preferably also develops anoutput 62 indicative of signal presence and applied according to thepresent invention to control a squelch control circuit generallydesignated at 64, which in turn functions to automatically operatesquelch control contact S2, such manner of control beingdiagrammatically designated in FIG. 1 by broken line 66. Said squelchcontrol circuit and the manner of control thereof by selection circuitoutput 62 are shown in more detail in FIG. 5 and discussed below inconnection therewith.

FIG. 2 is a block-schematic drawing of a portion of the passband segmentselection circuit 40, showing the components thereof by which thestrongest signalqoutput 42A is developed. In FIG. 2, the IF input SSMisfed to a parallel array of bandpass filters (BPF) D1, D2, Dn. Each ofthe bandpass filters D1, D2, Dn preferably has a passband substantiallyequal to the spectrum of the narrowband signal received by the receiver(such as a passband of 6 kcs. where the narrowband signal comprises acarrier modulated at i3 -kcs.), and the total number of bandpass filtersD1, D2, Dn is selected so that the bandpass filters collectively spanthe IF passband of the receiver. Thus, in the typical case illustratedat F-IG. 3, each of the bandpass filters D1, D2, Dn has a passband of 6kcs. (between -6 db points), and a total of six bandpass filters areemployed in the selected case Where the narrowband signal is modulatedat i3 kcs. and the IF passband of the receiver is 36 kcs. A fullillustration of this arrangement involving a total of six bandpassfilters would of course require a showing in FIG. 2 (and also in FIGS.4, 5 and 6 discussed below) of a total of six bandpass filters. However,since the branch circuitry employed with each of the bandpass filters isthe same, and since the total number of bandpass lfilters will be variedaccording to particular design considerations, the illustrations at FIG.2 et seq. show three of the bandpass filters, D1 being the first (lowestfrequency) bandpass filter, D2 being the second (next lowest frequency)bandpass filter, and Dn being the last (highest frequency) bandpassfilter making up the parallel array, with broken line connections to thecircuitry associated with filter Dn being used to show that additionallike filters and branch circuitry may be interposed.

The respective outputs 70, 72, 74 from filters D1, D2, Dn are fedthrough coupling capacitors 76, 78, to the cathodes of respective diodes82, 84, 86, with respective direct current (DC) return resistors 88, 90,92 being provided. The respective plates of the diodes 82, 84, 86 areall joined together so as to provide a common output at 94, resistor 96and IF shunt capacitor 98 providing a common load so that the strongestsignal output 42A is at audio frequency (AF), i.e. is a demodulatedsignal.

The strongest signal segment selection circuit shown at FIG. 2 functionsas follows. Assuming the strongest signal falls within a given bandpassfilter passband, say that of filter D1, the strongest IF wave is fed todiode 82 which, in conjunction with the common load 96, 98, demodulatesthe wave producing an AF wave across load resistor 96 as well as anegative DC voltage component in output 94. This negative DC' voltagecomponent back biases the other diodes 84, 86 and therefore signals ornoise com,- ponents falling within the passbands of their respectiveassociated filters D2, lDn are excluded from the output 94. Thus, thediodes `82, 84, 86 develop a single output and function as bothdemodulators and as gates, the gating action providing that the detectorassociated with the bandpass filter having highest energy level operatesto detect and pass that signal energy, while the other detector-gatesblock passage of signals from the other bandpass filters. The variousbandpass lters in effect function to separate the energy in the receiverpassband into spectraf segments, and the associated diodes function tocompare the relative energy levels of the energies at the varioussegments, and further function to select as an output only that energysegment (or possibly plural segments if the energy levels therein areessentially equal) as the detection stage output, i.e. the receiveroutput.

In some cases it is desirable to be able to select the next to strongestsignal in the receiver passband, to the exclusion of the strongestsignal, or to select an even weaker signal to the exclusion of strongersignals.

`Circuitry for selection of signals of various strengths, to theexclusion of other signals, is rshown schematically in FIG. A4. In thiscircuit, and in addition to the circuit components by which strongestsignal output 42A is developed as above discussed, a second set of diodedetection and gating means are employed which select and isolate thefilter output having the second largest energy level. 4In addition, asshown in FIG. 4, a third set of diode detection and gating means can beemployed to select and isolate a third largest or weakest filter output.In genral, the number of arrays of diode detection and gating ieans canbe equal to or less than the number of bandass filters D1, D2, Dn used;however, in practice only a tronger signal output 42A and a secondstrongest signal `utput 42B would be all the outputs normally required.

The second strongest output 42B is developed in the ircuit shown in FIG.4 in the following manner. By way if typical example an operationalcondition is assumed /here filter D1 is segregating the strongest signalat a iven instant and the next strongest signal is being segreated byfilter D2, the amplitude of the output from ,lter D1 being 20 v. RMS andthe amplitude of the outut from filter D2 being 10 v. RMS. Under thesecircumtances the DC bias produced by the diode 82 across )ad 9.6 is -16volts, and the DC current flowing through )C return resistor 88 producesa DC potential thereat `f +4 volts. Since the peak of the energy fromfilter D2 s less than the -16 volts produced across load resistor 6, nocurrent ows through diode 84 and therefore no )C potential is developedacross its associated DC return esistor 90. In the second strongestsignal selection circuit hown in FIG. 4, respective resistors 100, 102,104 and apacitors 106, 108, 110 are lowpass filters (LPF) which.ttenuate the IF by a suitable factor, say :1. Thus, asuming that the DCreturn bias at the input to lowpass llter 100, 106 is +4 volts, the backbias is suiiicient to ut off diode 112 since the attenuated IF signalappearng at the cathode of diode 112 has an amplitude of i2 olts R.M.S.The input to lowpass filter 102, 108, does tot include any back bias,since there is no current fiow hrough DC return resistor 90, and theoriginally $10 'olt RMS signal input to said lowpass filter 102, 108apears at the cathode of diode 114 as a signal having an `mplitude of ilvolt RMS. In the presence of this signal, nd without any DC back bias,diode 114 conducts and he demodulated wave therefrom appears at thecommon utput 118 across load resistor 120 and shunt capacitor .22, saidoutput 118 being the second strongest signal 2B. As will be apparent,the relatively strong output at 18 from conduction of diode 114 blocksany output rom diode 116, receiving a lesser strength signal fromandpass filter Dn through lowpass filter 104, 110, this onditionoccurring in the same manner as any outputs rom diodes 84, 86 areblocked by the output from diode l2 in the strongest signal selectionexample above disussed.

In a similar fashion, and as also shown in FIG. 4, a Weakest signaloutput 42u can also be selected, the seection circuit therefor includingsignal outputs 130, 132, .34 from the cathode sides of the diode arrayof the lrevious signal selection circuit. Assuming no internediatestages, said outputs become respective inputs 30', 132', 134 to therespective lowpass filters comlrising resistors 136, 138, 140 andcapacitors 142, 144, 46 to diodes 148, 150, 152, with the weakest signal.ppearing as the output 154 across load resistor 156 and hunt condenser158 because of back biasing of the other iodes 148, 150 in the circuitdiode 152 being conductive nd diodes 148, 150 being nonconductive inthis instance.

The passband segment selection circuit shown in FIG. also includes acontrol signal 62 to the squelch control ircuit shown at FIG. 5. Saidcontrol signal 62 comrises outputs, shown in FIGS. 4 and 5 at 160, 162,164, espectively, from the cathode side of respective diodes l2, 84, 86in the strongest signal selection circuit.

The principle of operation of the squelch control techtique of thepresent invention can best be understood ty first considering thespectrum characteristics of noise. `he types of noise experienced bycommunication ystems can be considered to be either thermal or shotnoise which is generally developed in resistors or ubes or transistors,or impulse noise which is generted in ignition or other forms of rotaryelectrical equipnent. In the case of thermal or shot noise, which isrlso known as white noise, the noise may be considered to be produced byan extremely large number of individual noise generators and theoryindicates that for frequencies generally used for communicationspurposes the spectrum distribution of the energy involved in this typeof noise is uniform, or essentially so. The other classification ofnoise is impulse noise, the spectrum of which is composed of lines thatare spaced at harmonics of the repetition rate at which the noise isgenerated. Generally, the width of the noise pulse is very short so thatnoise energy of this type is encountered even in the VHF and UHFfrequency ranges. Thus, even for impulse noise of the type generallyencountered in mobile communications operations, the noise within areceive or filter having a bandwidth of a few thousand cycles, more orless, can be considered to be essentially uniform.

The general uniformity of the noise energy distribution within thepassband of a wideband receiver is the underlying basis of the squelchcontrol system of the present invention.

FIG. 5 illustrates a simplified block-schematic diagram of such asquelch control system. The intermediate frequencypassband is separatedinto frequency segments as above described, by use of bandpass filtersD1, D2, Dn, and the outputs of said bandpass filters are fed to therespective detection and gating diodes 82, 84, 86, with a positive DCvoltage being generated across whichever DC diode return resistor 88,90, 92 is associated with the conductive diode. If only noise is beingreceived at any given time, then the output energy from each of thebandpass filters D1, D2, Dn is of a low order and essentially equal tothe energy in the other filter outputs, with the result that the averagecurrents flowing through each of the diode return resistors 88, 90, 92are substantially equal and are of such a low value in each instancethat the input is insufiicient to cause the squelch relay controlcircuit to operate. The squelch relay control circuit comprises therespective lowpass filters formed by.

resistances 166, 168, and capacitors 172, 174, 176 and respective diodes178, 180, 182 and vacuum tubes 184, 186, squelch control relay 188 beingthe plate load of tube 186. However, in the case where a signal isreceived in one of the bandpass filters D1, D2, Dn, and furthermoreassuming that the amplitude of the signal is large enough to make thelevel of the output from one of the filters D1, D2, Dn substantiallygreater than the outputs of the other filters, then the associated diode82, 84, or 86 becomes conductive, cutting ofi the other diodes, with theresult that all of the current passing through the output load resistor96 passes through but one of the return resistors 88, 90, 92, generatingenough increase in voltage at the grid of tube 184 to make normallynonconductive tube 184 conductive and normally conductive tube 186nonconductive, deenergizing squelch relay 188 and by `mechanical linkage66 changing the position of relay contact S2 (FIG. 1), establishing saidrelay contact S2 in the receiver sensitive position. Variable cathodeand plate load resistances 190, 192 in circuit with tube 184 provideadjusting means for the squelch activation level.

In FIG. 5, the various diodes 178, 180, 182 function to avoid anaveraging of the DC voltage, and accomplish this result by isolating thegrid of tube 184 from all of the other diode diversity circuits becauseof the back biasing of these other diodes, since their respectivelyassociated diodes 82, 84, 86 are nonconductive in the situation where arelatively strong signal (i.e. energy level) exists in but one of thebandpass filters D1, D2, Dn. Thus, when the filter outputs aresufficiently different in energy level to make only one of the diodes82, 84, 86 conductive enough to back bias and cut off the other diodes,the circuit shown at FIG. 5 detects the difference between signal leveland noise level and a signal-over noise type squelch control isrealized. Also, it is to be noted that the signal-tonoise ratio ofwhichever individual bandpass filter output is used to control squelchis considerably better than the signal-to-noise ratio of the total IFpassband. For example, if ten bandpass filters are used and assuming thenarrowband signal falls entirely within one filter passband, thesignal-to-noise ratio of the individual bandpass lter output is dbbetter than the signal-to-noise ratio of the entire IF passbandconsidering thermal or shot noise. In the case of impulse noise, thegain is db for a ten bandpass filter segment selection system. Withsignal-to-noise improvement of this order, it is relatively easy todetect signal presence, and to effect squelch control accuratelyresponsive to signal presence.

The present invention also has significant utility with regard toimprovement of performance of radio telegraph data transmission systems.In this respect, and by way of further example, FIG. 6 illustrates theuse of the invention in a frequency shift keying (FSK) typeradiotelegraph receiver. In a conventional signal FSK channel receiverinvolving a transmission rate of 60 to 100 words per minute, arelatively large frequency shift is normally used, the extent offrequency shift being on the order of 300 to 1000 c.p.s. For optimumsignal-to-noise ratio in the output, under poor conditions insofar asinput signalto-noise ratio is concerned, the amount of shift should beon the order of 85 c.p.s., but because the receiving equipment must beable to accommodate large amounts of drift as is prevalent inconventional FSK receiving and transmitting equipment, much widerfrequency shifts are used. By automatically selecting only that segmentof the IF passband which contains the signal at any given instant, thepresent invention alleviates this problem by allowing the FSK receiverto respond to the signal over a relatively wide frequency range but withonly a relatively narrow response with respect to noise and interferenceenergies.

As shown at FIG. 6, the IF section output of an otherwise conventionalFSK receiver is fed as input 200 to a parallel array of bandpass filtersD1', D2', Dn', thence through respective coupling capacitors 76', 78',80', and across respective DC return resistances 88', 90', 92 to thecathodes of diodes 82', 84', 86', the plates of the latter beingconnected together to provide a common output 94 across load resistor96. Whichever of the diodes 82', 84', 86 is being maintained conductive(by its associated lter D1', D2', Dn', having the signal present thereinat any given time), the `conductive 'diode functions to load the commonload resistor 20 and cut off the other of the diodes 82', 84', 86.Accordingly, only the signal and noise from the bandpass filter D1',D2', Dn passing the most energy is fed to the output load 96. A seriesresonant circuit composed of capacitor 202 and inductance 204 selectsthe IF component of the energy loading output load 96 of the segmentselector circuit, and the IF input1 206 thus selected if fed toamplitude limiter 208 Which removes amplitude modulation noise andprovides an input 210 to the wideband discriminator 212. Said widebanddiscriminator 212 responds to any IF wave passed by any of the bandpasslters D1', D2', Dn and the output 214 from wideband discriminator 212 isa keying wave having a characteristic frequency separation between markand space frequencies.

In order to operate the associated teleprinter, it is necessary todetermine at any instant whether a mark or space is being transmitted.Assuming that a more positive voltage is produced in the output 214 fromthe discriminator 212 if a mark is being received and a less positivevoltage if a space is being received, then the automatic thresholdadjust circuit 216 produces a voltage input 218 to a threshold circuit220 which is an average of the mark and space voltages. For example, ifunder certain frequency 'drift conditions filter D1 is active and thediscriminator 212 produces +10 v. for mark signals and -2 v. for spacesignals, the automatic threshold adjust circuit would produce an averagesignal output 218 at +4 v. If the equipment drifts so that lter D2 isactive 10 and the discriminator 212 produces a signal at +8 v. for marksand a signal at -4 v. for space signals, the automatic threshold adjustcircuit 216 would produce an output 218 at +2 v.

FIG. 7 illustrates a schematic of the automatic threshold adjust circuit216. A portion 222 of the discriminator output 214 is fed to two diodes224, 226. Diode 224 is connected so that negative pulses are peakdetected and diode 226 provides peak detection of positive pulses. Thecurrent flow path for diode 224 is through return resistor 228, resistor230, and finally resistor 232, a voltage being produced across resistor232 which is a function of the peak amplitude of the negative pulse fedto the threshold adjust circuit. Capacitor 234 is large enough to makethe circuit function as a peak detector. Similarly, the current flowpath for positive pulse peak Idetection diode 226 includes returnresistor 228, resistor 236 and resistor 232, with capacitor 238 beinglarge enough to make the circuit function as a peak detector. Thevoltages thus produced across resistor 232 are there averaged andprovide an output which is an arithmetic mean of the peak negativepulses and peak positive pulses. Capacitor 240 stores this average DCvoltage over a long enough period so that the voltage does not followthe keying but rather the average voltage. In this manner the desiredcentering for the threshold circuit 220 is automatically maintained.Threshold circuit 220 is thus biased by adjust circuit 216 so as to beable to distinguish the mark voltages and space voltages, and the output242 therefrom feeds DC amplilier 244 which in turn functions to key theteleprinter 246 or other utilization device.

In the FSK circuit shown in FIG. 6, it is to be again noted that aconsiderable improvement in signal-to-noise ratio is obtained and thatthe signal-to-noise ratio of the output signal is essentially that ofthe best narrowband signal in the receiver passband (i.e. the system ineffect provides the same signal-to-noise advantage as would be providedby a narrowband FSK system), without any requirement of high frequencystability in either the FSK transmitter or the FSK receiver.

From the foregoing, various modifications and other adaptations of theinvention, or certain aspects thereof, will be apparent to those skilledin the art to which the invention is addressed, within the scope of thefollowing claims.

What is claimed is:

1. In a communications receiver having an intermediate frequencypassband substantially wider than the bandwidth of the signal receivedby the receiver, the improvement comprising:

(a) a plurality of bandpass means Separating the energies in theintermediate frequency passband of the receiver into a plurality offrequency segments, each such bandpass means having a passband aboutequal to the bandwidth of the received signal;

(b) gating means respectively comparing the energy levels in each ofsuch passband means and passing only that passband energy having aselected energy level different from the energy levels in other of thepassband means; and

(c) means utilizing the gated energy as the receiver output.

2. A communications receiver according to claim 1, wherein said gatingmeans passes only the bandpass energy wherein the energy level isstrongest,

3. A communications receiver according to claim 1, wherein said gatingmeans passes only the bandpass energy 'wherein the energy level is lessthan the energy level in another bandpass means.

4. A communications receiver according to claim 1, wherein said gatingmeans passes only the bandpass energy wherein the energy level is oflesser strength than the energy levels in a plurality of other bandpassmeans.

[5. vIn a communications receiver having an intermediate frequencypassband, and receiver output means in- 11 luding a squelch circuitfunctioning to maintain the reeiver fully sensitive only when thereceived signal has at east a predetermined energy level, theimprovement comnrising:

(a) a plurality of bandpass means separating the energies in theimmediate frequency passband of the receiver into a plurality offrequency segments;

(b) gating means respectively comparing the energy levels in each ofsuch passband means and providing a squelch control signal only when asubstantial difference exists in the respective energy levels in suchbandpass means; and

(c) means applying such squelch control signal to said squelch circuit][6. A communications receiver wherein a received siglal occupies anintermediate frequency passband subtantially wider than the bandwidth ofthe received siglal, and wherein a squelch circuit functions to controleceiver output responsive to signal strength, the imirovementcomprising:

(a) a plurality of bandpass means separating the energies of theintermediate frequency passband of the receiver into a plurality offrequency segments, each such passband means having a passband aboutequal to the bandwidth of the received signal;

(b) gating means respectively comparing the energy levels in each ofsuch bandpass means and passing only that bandpass energy having aselected energy level different from the energy levels in the other ofthe bandpass means; and

(c) receiver output means, including said squelch circuit, responsive tothe gated bandpass energy, with the selected energy operating suchsquelch circuit to render the receiver fully sensitive only when theselected energy is substantially greater than the energy levels in suchother bandpass means] 7. A communications receiver according to claim 6,vherein said squelch circuit is responsive to the bandpass ,nergy havingthe greatest energy level.

8. A communications receiver according to claim 6, :omprising at leastthree bandpass means.

9. In a wideband radio receiver used to receive a narow band signalwherein the receiver comprises radio requency amplification means, andintermediate freluency amplification section, detection means, and re-:eiver output means, the improvement comprising:

(a) a plurality of bandpass means separating the energies in theintermediate frequency passband into various frequency related passbandsegments, each occupying a frequency spectrum about equal to thefrequency spectrum of the received narrowband signal;

(b) gating means respectively comparing the energy levels of the variouspassband segments and selecting and detecting only the energy in onesuch bandpass means While blocking the energy in the other such bandpassmeans; and

(c) means utilizing the gated energy as the receiver output.

10. A communications receiver according to claim 9, 'urther comprising asquelch circuit, and wherein such gated energy controls the squelchcircuit to render said 'eceiver fully sensitive only when the selectedenergy 'rom one bandpass means is substantially greater than he energylevels in at least some of the other bandpass neans.

11. In a Wideband radio receiver used to receive nar- Iowband signalswherein the receiver comprises radio requency amplification means,wideband intermediate requency amplication section, detection means, andLudio frequency amplification means, the improvement :omprising aparallel array of bandpass filters separatng the energy in theintermediate frequency passband nto spectral segments, each of saidbandpass iilters havng a passband substantially equal to the bandwidthof said narrowband signal, means comparing the energy levels of thevarious said spectral segments, and means detecting and selecting onlythe energy in part of said spectral segments as the input to said audiofrequency amplification means.

12. A wideband receiver according to claim 11, comprising at least threebandpass lters, each having a passband substantially equal to thebandwidth of said narrowband signal.

13. A wideband receiver according to claim 12, wherein each passbandfilter has an eective passband of about six kilocycles.

14. A wideband receiver according to claim 13, wherein the passband ofsaid intermediate frequency amplification section is about thirty-sixkilocycles with six said bandpass filters collectively spanning saidintermediate frequency passband.

15. A wideband radio receiver used to receive a narrowband signalcharacterized by keyed carrier shift for data transmission, the signalpath in said receiver cornprising radio frequency amplification means, awideband intermediate frequency amplication section, means separatingthe energy in the intermediate frequency passband into various frequencyrelated segments, means comparing the energy levels of the various saidfrequency related segments and producing as an intermediate frequencyoutput only the energy in the frequency segment having the highestenergy level, amplitude limiter means removing amplitude modulationenergy from said intermediate frequency output, and widebanddiscriminator means converting said intermediate frequency output to -anoutput reflecting the keyed characteristics of said narrowband signal.

16. -A wideband radio receiver according to claim 15, characterized by agiven DC voltage output responsive to a given carrier frequency and byanother DC voltage output responsive to a shifted carrier frequency, thesaid receiver further comprising peak pulse detection meansautomatically maintaining the output from said Wideband discriminatormeans at an average value of zero volts.

17. In a wideband radio receiver used to receive relatively narrowbandelectromagnetic signals; a signal path comprising wideband intermediatefrequency amplification means; a parallel array of bandpass lters, eachoccupying a passband within the intermediate frequency of said receiverpassband, with said filters collectively spanning the said intermediatefrequency passband; a parallel array of diode detection means, eachreceiving the output from a different one of said bandpass iilters;means combining the outputs from said diode detection means so that inthe event of any substantial difference in energy level of therespective energies passed by the respective bandpass filters, the diodedetection means associated with that bandpass filter having the lhighestenergy level operates to detect and pass signal energy, while the otherdiode detection means block passage of signals from the other bandpassiilters.

18. In a communications receiver having a passband, and receiver outputmeans including a squelch circuit functioning to maintain the receiverfully sensitive only when the received signal has at least apredetermined energy level, the improvement comprising:

(la) a plurality of bandpass means separating the energies in thepassband of the receiver into a plurality of frequency segments;

(b) gating means respectively comparing the energy level in each of suchpassband means individually with the energy levels in every other suchpassband means and providing a squelch control signal only when asubstantial dierence exists in any one of the respective energy levelsin such bandpass means; und

(c) means applying suc/t squelch control signal to said squelch circuit.

13 19. A communications receiver having an input passband which issubstantially wider than the bandwidth of a received signal, and whereina squelch circuit functions to control receiver output responsive tosignal strength, the improvement comprising:

(a) a plurality of bandpass means separating the energies of thepassband of the receiver into a plurality 09 frequency segments, eachsuch passband means having a passband about equal to the bandwidth ofthe received signal;

(b) gating means respectively comparing the energy levelsl in each ofsuch bandpass means and passing only that bandpass energy having aselected energy level different from the energy levels in the other ofthe bandpass means; and

(c) receiver output means, including said squelch circuit, responsive tothe gated bandpass energy, with the selected energy operating suchsquelch circuit to render the receiver fully sensitive only when theselected energy is substantially greater than the energy levels in suchother bandpass means.

References Cited The following references, cited by the Examiner, are ofrecord in the patented le of this patent or the original patent.

UNITED STATES PATENTS 2,923,814 2/1960 Smith-Janiz, Jr 325-477 3,044,0187/1962 Wilson 325-320 3,112,452 11/1963 Kirkpatrick 325-479 3,126,4493/1964 Shirman 325-474 RICHARD MURRAY, Primary Examiner U.S. C1. X.R.

